Systematic Studies on Homo- and Heteronuclear Doubly Bonded Compounds of Heavier Group 15 Elements

The first stable phosphabismuthene (R 1 --P=Bi--R 2 ) and stibabismuthene (R 1 --Sb=Bi--R 2 ) were successfully synthesized by taking advantage of efficient steric protection groups, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt), 2,6-bis[bis(trimethylsilyl)-methyl]-4-[tris(trimethylsilyl)methyl]phenyl (Bbt), and 2,4,6-tri- t -butylphenyl (Mes*). Their spectroscopic properties and structural parameters were systematically compared with those of previously reported stable, homonuclear, doubly bonded systems, such as diphosphene, diarsene, distibene, and dibismuthene.     

Doubly bonded systems between heavier Group 15 elements

There has been much interest in the synthesis and properties of doubly bonded systems between heavier Group 15 elements, i. e. heavier analogues of azo-compounds (dipnictenes), from the viewpoints of fundamental and material chemistry. Although such double-bond compounds between heavier main group elements are known to be highly reactive, too much so to be isolated as stable compounds, a number of reports on the synthesis of kinetically stabilized diphosphenes (RP[double bond, length as m-dash]PR), diarsenes (RAs[double bond, length as m-dash]AsR), and phosphaarsenes (RP[double bond, length as m-dash]AsR) bearing bulky substituent have been published since 1980. We have also succeeded in the synthesis of the first stable distibene (RSb[double bond, length as m-dash]SbR) and dibismuthene (RBi[double bond, length as m-dash]BiR) by taking advantage of efficient steric protection groups, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) and 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl (Bbt), and revealed their structures and properties systematically. Thus, the doubly bonded compounds between heavier Group 15 elements are no longer imaginary species but are those with real existence which are stable, even in the case of the heaviest non-radioactive element bismuth, when they are appropriately protected by bulky substituents. This Perspective describes our research on the chemistry of kinetically stabilized double-bond compounds between heavier Group 15 elements.     

[2+3]Cycloaddition reaction of a kinetically stabilized distibene with a nitrile oxide leading to the formation of a unique heterocyclic compound

The synthesis of a 1-oxa-5-aza-2,3-distibacyclopent-4-ene derivative by the [2+3]cycloaddition reaction of a kinetically stabilized distibene, BbtSb=SbBbt (Bbt = 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris-(trimethylsilyl)methyl]phenyl), with MesCNO (Mes = mesityl) has been performed. Dedicated to Prof. Dr. E. Lukevics on the occasion of his 70th birthday __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1880–1887, December, 2006.     

Synthesis and properties of a new kinetically stabilized digermyne: new insights for a germanium analogue of an alkyne

The reduction of an overcrowded (E)-1,2-dibromodigermene, Bbt(Br)Ge=Ge(Br)Bbt (2) [Bbt = 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl], with KC8 afforded a stable digermyne, BbtGe[triple bond]GeBbt (1). The Ge[triple bond]Ge triple-bond characters of 1 were revealed by the X-ray crystallographic analysis and spectroscopic studies (UV/vis and Raman spectra) together with theoretical calculations. The Ge[triple bond]Ge bond lengths of the two nonidentical molecules of 1 observed in the unit cell were shorter than that of the previously reported digermyne, Ar'Ge[triple bond]GeAr' (Ar' = 2,6-Dip2C6H3, Dip = 2,6-diisopropylphenyl).     

Generation, isolation, and reactivity of a kinetically stabilized diphosphene anion radical

The stable lithium diphosphene anion radical, [Li(dme)₃]⁺[TbtPPTbt]⁻ (dme: 1,2-dimethoxyethane, Tbt: 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl), was readily synthesized by the one-electron reduction of the corresponding neutral diphosphene (TbtPPTbt). The molecular structure of the diphosphene anion radical was discussed in detail on the basis of its ESR, UV-Vis and Raman spectra, and theoretical calculations. The diphosphene anion radical was found to undergo ready chalcogenation reactions using elemental sulfur and selenium to afford the corresponding thiadiphosphirane and selenadiphosphirane, respectively.     

1,2,5]Thiadiazolo[3,4-c][1,2,5]thiadiazolidyl: a long-lived radical anion and its stable salts

[1,2,5]Thiadiazolo[3,4-c][1,2,5]thiadiazole (1) is synthesized in 62% yield by fluoride ion-induced condensation of 3,4-difluoro-1,2,5-thiadiazole with (Me(3)SiN=)(2)S. The reversible electrochemical reduction of 1 leads to the long-lived [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazolidyl radical anion (2) and further to the dianion (3). The radical anion 2 is also obtained by the chemical reduction of the precursor 1 with t-BuOK in MeCN. The radical anion 2 is characterized by ESR spectroscopy in solution and in the crystalline state. The stable salts [K(18-crown-6)][2] and [K(18-crown-6)][2].MeCN (8 and 9, respectively) are isolated from the spontaneous decomposition of the [K(18-crown-6)][PhXNSN] (6, X = S; 7, X = Se) salts in MeCN solution followed by XRD characterization. The radical anion 2 acts as a bridging ligand in 8 and as chelating ligand in 9. The structural changes observed by XRD in going from 1 to 2 are explained by means of DFT/(U)B3LYP/6-311+G calculations.     

Chemical reduction of 2,4,6-tricyano-1,3,5-triazine and 1,3,5-tricyanobenzene. Formation of novel 4,4',6,6'-Tetracyano-2,2'-bitriazine and its radical anion

Chemical reduction of 2,4,6-tricyano-1,3,5-triazine, TCT, results in the formation of an unstable radical anion that undergoes immediate dimerization at a ring carbon to form [C(12)N(12)](2-), [TCT](2)(2-), characterized by a long 1.570 (4) A central C[bond]C. [TCT](2)(2-) can decompose into the radical anion of 4,4',6,6'-tetracyano-2,2'-bitriazine, [TCBT]*-, the one-electron reduced form of planar (D(2h)) TCBT, which is also structurally characterized as the [TMPD][TCBT] charge-transfer complex (TMPD = N,N,N',N'-tetramethyl-p-phenylenediamine) with a 1.492 (2) A central sp(2)[bond]sp(2) C[bond]C. Although crystals could not be obtained for the radical anion [TCBT]*-, the electrochemistry (E degrees = +0.03 V), EPR (g = 2.003, (2)A((14)N) = 3.347 G, and (4)A((14)N) = 0.765 G and a line width of 0.24 G), and theoretical calculations support the formation of [TCBT]*-. In addition, thermolysis of [TCT](2)(2-) yields [TCBT]*-. Chemical reduction of 2,4,6-tricyanobenzene, TCB, forms an unstable radical anion that immediately undergoes dimerization at a ring carbon to form [C(12)H(6)N(6)](2-), [TCB](2)(2-), which has a long 1.560 (5) A central C[bond]C. Reaction of TCT with tetrathiafulvalene (TTF) forms structurally characterized [TTF][TCT], and in the presence of water, TCT hydrolyzes to 2,4-dicyano-6-hydroxy-s-triazine, DCTOH. In contrast, the reaction of TCT with TMPD forms [TMPD][TCT], which in the presence of water forms structurally characterized [HTMPD](+)[DCTO](-).     

Behavior of electrogenerated bases in room-temperature ionic liquids

The reductive electrochemistry of substituted benzophenones in the aprotic room-temperature ionic liquid (RTIL) 1-butyl-1-methylpyrrolidinium bistriflimide occurs via two consecutive one-electron processes leading to the radical anion and dianion, respectively. The radical anion exhibited electrochemical reversibility at all time-scales whereas the dianion exhibited reversibility at potential sweep rates of >or=10 V s(-1), collectively indicating the absence of strong ion-paring with the RTIL cation. In contrast, reduction in 1-butyl-3-methylimidazolium bistriflimide is complicated by proton-transfer from the [Bmim] cation. At low potential sweep rates, reduction involves a single two-electron process characteristic of either an electrochemical, chemical, electrochemical (ECE) or disproportion-type (DISP1) mechanism. The rate of radical anion protonation in [Bmim] is governed by basicity and conforms to the Hammett free-energy relation. At higher potential sweep rates in [Bmim][NTf2], reduction occurs via two consecutive one-electron processes, giving rise to the partially reversible generation of the radical anion and the irreversible generation of the dianion, respectively. Also, the redox potentials for the reversible parent/radical anion couples were found to be a linear function of Hammett substituent constants in both RTIL media and exhibited effectively equivalent solvent-dependent reaction constants, which are similar to those for reduction in polar molecular solvents such as acetonitrile or alcohols.     

A new class of chiral organogermanes derived from C2-symmetric dithiols: synthesis,characterization and stereoselective free radical reactions

A new class of dithiostannanes and dithiogermanes have been prepared from 1,1'-binaphthyl-2,2'-dithiol and 3,3'-bis(trimethylsilyl)-1,1'-binaphtho-2,2'-dithiol. While reduction of 4-butyl-4-chloro-3,5-dithia-4-stanna-cyclohepta[2,1-a;3,4-a']dinaphthalene to the corresponding tin hydride was unsuccessful, 4-tert-butyl-3,5-dithia-4-germa-cyclohepta[2,1-a;3,4-a']dinaphthalene and 4-tert-butyl-2,6-bis(trimethylsilyl)-3,5-dithia-4-germa-cyclohepta[2,1-a;3,4-a']dinaphthalene were obtained by reduction of the parent germanium chlorides with NaBH(4) and LiBH(4), respectively. Kinetic constants for hydrogen transfer to a primary alkyl radical were measured for both germanium hydrides. Reduction of alpha-halo carbonyl compounds by these germanium hydrides occurs with moderate ee values (up to 42%), while hydrogermylation of methyl methacrylate occurs with low selectivity (<3/1) for the former hydride but high selectivity (>10/1) for the latter.     

Nickel(II)-phenoxyl radical complexes: structure-radical stability relationship

Nickel(II) complexes of N3O-donor tripodal ligands, 2,4-di-tert-butyl-6-[([bis(2-pyridyl)methyl]amino)methyl]phenol (HtbuL), 2,4-di-tert-butyl-6-[([(6-methyl-2-pyridyl)methyl](2-pyridylmethyl)amino)methyl]phenol (HtbuLMepy), and 2,4-di-tert-butyl-6-[([bis(6-methyl-2-pyridyl)methyl]amino)methyl]phenol (HtbuL(Mepy)2), were prepared, and [Ni(tbuL)Cl(H2O)] (1), [Ni(tbuLMepy)Cl] (2), and [Ni(tbuL(Mepy)2)Cl] (3) were structurally characterized by the X-ray diffraction method. Complexes 1 and 3 have a mononuclear structure with a coordinated phenolate moiety, while 2 has a dinuclear structure bridged by two chloride ions. The geometry of the Ni(II) center was found to be octahedral for 1 and 2 and 5-coordinate trigonal bipyramidal for 3. Complexes 1-3 exhibited similar absorption spectra in CH3CN, indicating that they all have a mononuclear structure in solution. They were converted to the phenoxyl radicals upon oxidation with Ce(IV), giving a phenoxyl radical pi-pi transition band at 394-407 nm. ESR spectra at low temperature and resonance Raman spectra established that the radical species has a Ni(II)-phenoxyl radical bond. The cyclic voltammograms showed a quasi-reversible redox wave at E1/2=0.46-0.56 V (vs Ag/AgCl) corresponding to the formation of the phenoxyl radical, which displayed a first-order decay with a half-life of 45 min at room temperature for 1 and 26 and 5.9 min at -20 degrees C for 2 and 3, respectively. The radical stability increased with the donor ability of the N ligands.     

Radical ions : XXVII. Radical ions of tetrakis(trimethylsilyl)butatriene

One-electron oxidation and one-electron reduction of the electron-rich acetylene derivative, hexakis(trimethylsilyl)-2-butyne [H₃C₃)₃Si]₃CCCC[Si(CH₃)₃]₃, unexpectedly produce the persistent radical cation and radical anion of the hitherto unknown neutral compound, tetrakis(trimethylsilyl)butatriene (R₃Si)₂CCCC(SiR₃)₂. The radical anion can also be generated from the corresponding diacetylene, bis(trimethylsilyl)-1,3-butadiyne R₃SiCCCCSiR₃ and potassium metal, obviously via disproportionation. Photoelectron and electron spin resonance spectroscopic data permit the detection and characterization of the individual species. The stability of both the radical anion and the radical cation of the same molecule can be rationalized by the unique combination of the twofold butatriene π-system with 4 R₃Si substituents, which can act either as electron donors or electron acceptors and thus stabilize the ground state of either the cation or the anion.     

Electrochemical behavior of nickeladithiolene S,S'-dialkyl adducts: evidence for the formation of a metalladithiolene radical by electrochemical redox reactions

The electrochemical behavior of nickeladithiolene S,S'-dialkyl adducts (alkyl = benzyl, methyl, tert-butyl) was investigated by using cyclic voltammetry (CV), visible, near-IR, and ESR spectroscopies and bulk electrolyses. The redox potentials of the S,S'-dialkyl adducts were influenced by the electron-donating effect of the functional group on the sulfur atoms. The nickeladithiolene S,S'-dibenzyl adduct [Ni[S(SCH(2)Ph)C(2)Ph(2)](2)] (2) eliminated one benzyl radical by one-electron reduction, and then the monobenzyl adduct anion [Ni(S(2)C(2)Ph(2))[S(2)(CH(2)Ph)C(2)Ph(2)]](-) (3(-)) was formed. Anion 3(-) was also formed by the reaction of nickeladithiolene dianion [Ni(S(2)C(2)Ph(2))(2)](2)(-) (1(2-)) with 1 equiv of benzyl cation. When anion 3(-) was oxidized, the long-lived nickeladithiolene radical [Ni(S(2)C(2)Ph(2))[S(2)(CH(2)Ph)C(2)Ph(2)]] (3) was formed. The visible, near-IR, and ESR spectra of radical 3 could be measured and assigned. When radical 3 was further oxidized, the oxidant 3(+) eliminated one benzyl cation, and then free nickeladithiolene (1) was generated.     

Preparation, properties, and reduction of a novel TCNQ-type thienoquinoid

[Structure: see text] TCNQ-type bithienoquinoid condensed with a alpha-dicarbonyl group was synthesized. The compound revealed a reversible reduction wave at +0.32 V vs SCE. The radical anion species was cleanly generated by a reduction with Na-Hg and characterized: lambdamax=1027 nm in vis-NIR and a large spin density of the beta-carbon atoms on the fused ring by EPR spectroscopy. The radical anion salts were isolated by a reduction with iodide and showed moderate conductivity.     

The Zintl ion [As7]2-: an example of an electron-deficient As(x) radical anion

[K(2,2,2-crypt)](2)[As(7)]·THF, 1 (2,2,2-crypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) is the first well characterized seven-atom radical anion of group 15. UV-Vis spectroscopy confirms the presence and electronic structure of [As(7)](2-). Cyclic voltammetry in DMF solution shows the As(7)(3-)/As(7)(2-) redox couple as a one-electron reversible process. Theoretical investigations explore the bonding and properties of compound 1.     

Double-layer effects and distance dependence of electron transfer in reduction of nitro aromatic radical anions

The first example of the effect of an electric double layer on the reduction of electrochemically generated radical species is reported. The anion radical of methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate (pesticide bifenox) is electrochemically reduced in acetonitrile to a phenylhydroxylamine derivative in a process involving three electrons. This heterogeneous reaction is strongly influenced by the concentration and nature of the cation of the indifferent electrolyte. Depending on the type of tetraalkylammonium cation, the redox potential changes by 0.45 V. The kinetic parameters were obtained for five tetraalkylammonium hexafluorophosphate salts. The Frumkin correction, which assumes that the outer Helmholtz plane coincides with the reaction site, was applied to kinetic data of the radical anion reduction. The correction of the apparent rate accounted for the observed effect only in the case of tetramethylammonium salt. The presence of higher tetraalkylammonium homologues causes deviations from the predicted dependence of the electron-transfer rate on the phi2 potential of the outer Helmholtz plane. Hence, the nature of the cation of the electrolyte exerts a further effect extending beyond the electrostatic repulsion only. The corrected rate of electron transfer decreases exponentially with increasing size of the alkyl chain of the indifferent electrolyte cation in the order methyl > ethyl > propyl > butyl > hexyl. The rate decay is characterized by an exponent beta = 0.83. This confirms that the reaction plane for the reduction of the bifenox radical anion is different for each electrolyte. Due to this fact the Frumkin correction cannot fully account for the observed dependence of the heterogeneous rate on the solution composition. The observed effect is not specific to the bifenox radical. A similar influence of the concentration and nature of the cation of the indifferent electrolyte was observed for other nitro compounds, namely, nitrobenzene, nitrobenzoate, and nitrofen.     

Radical anion of isolable dialkylsilylene

By the reduction of an isolable dialkylsilylene, 2,2,5,5-[tetrakis(trimethylsilyl)]-1-silacyclopentane-1,1-diyl (1), with cesium, rubidium, potassium, sodium, and lithium 4,4'-di(tert-butyl)biphenylide in DME at low temperatures, the corresponding silylene radical anion 2 was generated as the first persistent silylene radical anion in solution and characterized by ESR spectroscopy. Radical anion 2 is rather stable at -70 degrees C in DME but decomposes rapidly at room temperature with a half-life time of ca. 20 min. The g-factor and 29Si hyperfine splitting constants (hfs's) of 2 are almost independent of the countercations, indicating that 2 exists as a free ion or a solvent-separated ion pair in a polar DME solution. A very small hfs due to the 29Si nucleus of the divalent silicon (3.0 mT) as well as a very large g-factor (2.0077) indicates that an unpaired electron is accommodated in the vacant 3ppi orbital of silylene 1.     

When ethyl is infinitely different from methyl: double addition of lithiated dithianes to aromatic carboxylates revisited

Addition of lithiated alkyl dithianes to benzoyl chloride or methyl benzoate does not produce the expected product of double addition, alpha,alpha-bis(alkyldithianyl) benzyl alcohol, for alkyls larger than methyl. Instead, the first step intermediate, i.e. 2-benzoylated dithiane, undergoes an electron-transfer reduction by the second molecule of the dithianyl anion. This reduction is followed by the ring-opening mesolytic fragmentation of the dithiane ring in the ketyl anion radical and subsequent radical recombination yielding acetophenone-tethered thioortho esters 4, alpha-[3-(2-alkyl-1,3-dithiane-2-ylthio)propylthio]-alpha-alkyl-acetophenones. It appears that the Corey-Seebach bisaddition of lithiated dithianes to methyl benzoate is an exception rather than the rule in the alkyl dithiane series.     

Formation of a 1-bicyclo[1.1.1]pentyl anion and an experimental determination of the acidity and C-H bond dissociation energy of 3-tert-butylbicyclo[1.1.1]pentane

Decarboxylation of 1-bicyclo[1.1.1]pentanecarboxylate anion does not afford 1-bicyclo[1.1.1]pentyl anion as previously assumed. Instead, a ring-opening isomerization which ultimately leads to 1,4-pentadien-2-yl anion takes place. A 1-bicyclo[1.1.1]pentyl anion was prepared nevertheless via the fluoride-induced desilylation of 1-tert-butyl-3-(trimethylsilyl)bicyclo[1.1.1]pentane. The electron affinity of 3-tert-butyl-1-bicyclo[1.1.1]pentyl radical (14.8 plus minus 3.2 kcal/mol) was measured by bracketing, and the acidity of 1-tert-butylbicyclo[1.1.1]pentane (408.5 +/- 0.9) was determined by the DePuy kinetic method. These values are well-reproduced by G2 and G3 calculations and can be combined in a thermodynamic cycle to provide a bridgehead C-H bond dissociation energy (BDE) of 109.7 +/- 3.3 kcal/mol for 1-tert-butylbicyclo[1.1.1]pentane. This bond energy is the strongest tertiary C-H bond to be measured, is much larger than the corresponding bond in isobutane (96.5 +/- 0.4 kcal/mol), and is more typical of an alkene or aromatic compound. The large BDE can be explained in terms of hybridization.     

Photochemical isomerization of 1,2,5-trisilabicyclo[3.2.0]hepta-3,6-diene to 1,4,7-trisilabicyclo[2.2.1]hepta-2,5-diene

Upon irradiation of a benzene-d6 solution of 1,2,2,5-tetrakis[di-tert-butyl(methyl)silyl]-4,7-diaryl- 1,2,5-trisilabicyclo-[3.2.0]hepta-3,6-diene [1a: aryl = phenyl, b: aryl = 3,5-bis-(trimethylsilyl)phenyl], 1,4,7,7-tetrakis[di-tert-butyl-(methyl)silyl]-2,5-diaryl-1,4,7- trisilabicyclo[2.2.1]hepta-2,5-diene (2a,b) was formed via skeletal rearrangement.     

The Chemistry of Stable Silabenzenes

Stable silabenzenes ( 1a; R = Tbt, 1b; R = Bbt) were synthesized by taking advantage of extremely bulky and efficient steric protection groups, 2,4,6‐tris[bis(trimethylsilyl)methyl]phenyl (Tbt) and 2,6‐bis‐[bis(trimethylsilyl)methyl]‐4‐[tris(trimethylsilyl)methyl]phenyl (Bbt). The structure of Tbt‐substituted 1a was determined by X‐ray crystallographic analysis, which demonstrated the complete delocalization of the π‐electrons of the silabenzene ring. It was found that silabenzene 1a reacted with C–C and C–O multiple bond compounds to give the corresponding [4+2]‐cycloadducts via 1,4‐addition, while 1a underwent both 1,2‐ and 1,4‐additions by the reaction with methanol. Silabenzene 1a dimerized very gradually to afford its [4+2]‐dimer, although 1b showed no change under the same conditions. Photochemical reaction of 1a gave the corresponding silabenzvalene isomer instead of the Dewar silabenzene isomer.